The present invention relates to the field of scanned beam microscopy, and in particular, to a method and apparatus for calibration of a scanned beam system.
Scanned beam microscopy systems, including charged particle beam systems such as electron beam and focused ion beam (FIB) systems, are widely used in characterization or treatment of materials on a microscopic scale. For example, focused ion beam systems are used in manufacturing operations because of their ability to image, etch, mill, deposit and analyze with great precision. Ion columns in FIB systems using gallium liquid metal ion sources (LMIS), for example, can provide five to seven nanometer lateral imaging resolution.
The beam of a scanning beam system typically scans the surface of a target specimen in a raster pattern. This raster pattern may be used to produce an image of the surface of the target. When the scanned beam strikes the target, particles or photons are emitted from the immediate vicinity of beam impact. A portion of these emissions are measured or collected using a suitable detector or collector that produces an output signal indicative of the intensity of the emission. This output signal is then processed to produce an observable image displayed on a conventional video monitor.
A typical application of scanning beam systems is for analysis and treatment of integrated circuits (IC). In this application, a focused ion beam is used to produce an image of the circuit. This image is then used in conjunction with circuit layout information to navigate the ion beam over the surface of the circuit to locate a specific element or feature of interest. When the beam is scanned to the local area of interest, the beam current can be increased to cut into the circuit die and expose circuit features buried in layers. The FIB system can then alter the exposed circuit by cutting conductive traces to break electrical connections or by depositing conductive material to provide new electrical connections. This etching or deposition is caused by a physical or chemical reaction of the beam ions with the specimen and occurs at a rate that is largely dependent upon the constituent ions of the beam, the presence and type of etch enhancing or deposition precursor gases, and the beam current.
Also important in achieving accurate characterization and treatment of a specimen is the beam dwell time. The beam dwell time is the duration of time the beam dwells in a specific location on the specimen. In a scanned beam system, the beam is typically controlled by digital electronics to scan across the specimen in a stepwise fashion from point to point, dwelling for a pre-determined time at each point. The distance between the sample points at which the beam dwells is referred to as the pixel spacing or pitch. When imaging the surface, if the dwell time is too short for a given beam current, insufficient collection of emissions occurs to accurately characterize the surface at the dwell point. When this occurs, the displayed image will appear xe2x80x9cnoisyxe2x80x9d because of a low signal-to-noise ratio.
A focused ion beam, even at relatively low energy, will always cause some destructive etching of the specimen surface. Even an electron beam can alter the specimen, for example, through electron-beam induced chemical reactions that cause hydrocarbons residual in the vacuum chamber to stain the sample surface. Because a charged particle beam will invariably cause changes in the specimen surface, a long dwell time will alter the surface, thereby decreasing the accuracy of the surface characterization. Thus, careful control of the beam intensity and dwell time at each point in the scan is required.
Further, the beam must be accurately focused and compensated for aberrations to provide a useful image of the specimen surface for visual or automated analysis. In a conventional method for focusing the beam, a calibration specimen is prepared consisting of an etched region or region of deposited material to form a target of well-defined shape upon which to focus the beam. When the beam is properly focused, the target shape will appear on a visual display in high contrast to the surrounding specimen surface. Once accurate focus is achieved, the calibration specimen is removed and the specimen to be analyzed or treated is placed in the plane of focus.
Unfortunately, to obtain a finely detailed image of the calibration specimen suitable for achieving sharp focus and precise calibration, many closely spaced samples of the target must be taken. When the pixel spacing is less than the beam spot sizexe2x80x94typically defined as the beam diameter for which the beam drops to one-tenth of its maximum valuexe2x80x94the problem of specimen degradation is exacerbated by the resultant high ion dose at each sample point. This degradation occurs at a rate that is sufficiently high to interfere with beam calibration. Conversely, if the beam current or dwell time is reduced to avoid this, then the signal-to-noise ratio decreases, resulting in a poor image of the calibration specimen that is unsuitable for achieving sharp focus and precise beam calibration. Further, using conventional scanning methods, fine calibration to remove small errors is difficult to achieve.
Thus, there is a need for methods and systems to achieve accurate scanned beam system calibration that overcome these and other limitations of the prior art.
The present invention provides for accurate calibration of a scanned beam system that overcomes limitations of the prior art. According to the methods of the present invention, a calibration specimen comprising an array of targets is sampled with a sample spacing that is greater than the spacing between the targets and an image is reconstructed from the samples.
The present invention enables achievement of very sharp beam focus and highly precise calibration without substantial degradation of the calibration specimen caused by closely spaced sampling. Slower scan speeds may be employed which provide an image of high contrast because of improved signal-to-noise ratio. Because the reconstructed image is composed of points spread at relatively large distances across the calibration specimen, beam aberrations and alignment errors are magnified and can be more readily corrected than when prior art calibration techniques are employed.
Application of the aliased image scanning technique of the present invention will magnify the effect of rotational misalignment of the calibration specimen with respect to the scan axes of the beam, enabling easier detection and correction of rotational misalignment. Also, conditions giving rise to a non-orthogonal relationship between the x-y axes of the image produced by the system are also magnified and can therefore be more easily detected and corrected. Further, beam stigmation effects are magnified for easier detection and correction. The invention is particularly well suited for use with automatic focusing and other automatic beam adjustments because it is very clear when the proper focus and other compensations are achieved.
The foregoing has rather broadly outlined features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed herein may be readily utilized as a basis for modifying or designing other structures for carrying out many useful purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.